The present invention relates to the synthesis of deuterated methylene chloride, in general, and to the synthesis of deuterated methylene chloride under phase-transfer catalysis (P-T-C) conditions, in particular.
Dideuterio-methylene chloride (dichloromethane -d.sub.2 ; CD.sub.2 Cl.sub.2) has become increasingly important in industrial and research applications as a reagent for chemical reactions, as a solvent for inorganic and organic compounds in various NMR spectroscopy, and in other applications where the properties of methylene chloride are desirable but where its protons must be replaced by deuterium. In particular, CD.sub.2 Cl.sub.2 is being recommended for applications where CDCl.sub.3 or even CH.sub.2 Cl.sub.2 were formerly used. For example, because many inorganic compounds and complexes are more soluble in CD.sub.2 Cl.sub.2 than in CDCl.sub.3, it has been recommended that CD.sub.2 Cl.sub.2 be used as a solvent in place of CDCl.sub.3 in NMR spectroscopy. In addition, reports have indicated that CD.sub.2 Cl.sub.2 is less toxic to mammals than is CH.sub.2 Cl.sub.2 and for that reason it may be preferable to use CD.sub.2 Cl.sub.2 in certain applications.
Although CD.sub.2 Cl.sub.2 offers many advantages over other reagents, at the present there is not a method of preparation that provides high yields of a highly deuterated methylene chloride at a low cost. For instance, Atkinson et al. in Chem. Abstr. 1970, 72, 110766y disclose a method of deuteration of CH.sub.2 Cl.sub.2 to CD.sub.2 Cl.sub.2 that is carried out in a homogeneous solution; CH.sub.2 Cl.sub.2 in dimethyl sulfoxide is mixed with D.sub.2 O containing NaOD and refluxed 24 hours to provide methylene chloride containing 33% D. It was reported that this product could be further enriched to 42% D by repeating the process (recycling). In the absence of dimethyl sulfoxide, no exchange occurs.
In many applications, however, it is preferable to use methylene chloride having a content in excess of 99% D. To achieve such a high percent of D/H substitution by using the method of Atkinson et al. would require a number of enriching cycles at 24 hours of refluxing per cycle. Because of the number of recycles required in order to obtain methylene chloride having a D content in excess of 99%, the productivity of the process would be poor. Moreover, because methylene chloride is soluble in dimethyl sulfoxide, it is not feasible to remove the deuterated product (CD.sub.2 Cl.sub.2) by simple layer separation. Furthermore, their exchange fails in the absence of dimethyl sulfoxide. As a consequence, the cost of preparing CD.sub.2 Cl.sub.2 by the Atkinson et al. process would be relatively high.
Other methods for preparing deuterated methylene chloride have been reported in the chemical literature. Myers et al. (J. Chem. Phys. 1952, 20, 1420-1427) and Shimanouchi et al. (J. Mol. Spectrosc. 1962, 8, 222-235) describe the preparation of deuterated methylene chloride by heating chloroform-d (CDCl.sub.3) with metallic zinc in CH.sub.3 CO.sub.2 D. Leitch et al (Can. J. Chem. 1953, 351-356) describe treating CD.sub.2 O with PCl.sub.5 to yield CD.sub.2 Cl.sub.2. In the former method, the yield is poor and much of the costly CDCl.sub.3 and CH.sub.3 CO.sub.2 D are destroyed in the process. In the latter method, the required starting material (CD.sub.2 O) is very costly and difficult to prepare in anything other than small laboratory amounts.
Accordingly, a need has remained for an improved simple and economic method for preparing and isolating deuterated methylene chloride on a large scale.